1. The Field of the Invention
The present invention relates to a local interconnect in an integrated circuit, and particularly to a method of making a local interconnect structure situated on an active area within a semiconductor substrate, where the local interconnect has tungsten silicide and titanium nitride layers, or has a tungsten layer covered on opposite sides thereof with titanium nitride.
2. The Relevant Technology
Local interconnects are normally used when a metalization is not being used to electrically connect semiconductor devices. A local interconnect is used, for example, to put a conductive material in electrical contact with an active area in a semiconductor substrate. In the context of this document, the term "semiconductor substrate" is defined to mean any construction comprising semiconductive material, including but not limited to bulk semiconductive material such as a semiconductive wafer, either alone or in assemblies comprising other materials thereon, and semiconductive material layers, either alone or in assemblies comprising other materials. The term "substrate" refers to any supporting structure including but not limited to the semiconductor substrates described above.
Typical local interconnects comprise materials such as titanium nitride, titanium silicide and other kinds of low resistance materials such as refractory metal silicides. Typically, after the gates and transistors and other topographical structures are defined, a refractory metal film is formed thereover either by sputtering, also known as physical vapor deposition process, or by chemical vapor deposition (CVD). Sputtering titanium nitride film is characterized by low resistivity with poor step coverage. As a result, sputtered titanium nitride film is difficult to use in circuits and devices with vertically oriented topographies. Sputtering produces a titanium nitride film with a lower resistivity than CVD titanium nitride, but has a poorer step coverage over vertically oriented topographies.
In situations where a hard mask is needed in the formation of an interconnect structure, hard masks have typically been comprised of polysilicon or silica. In a contact etch, the hard mask need not be removed if the contact etch can etch through the hard mask and stop etching on an underlying local interconnect without compromising active areas. If the contact etch can not etch through the hard mask without deleterious effect, then the hard mask should be removed during fabrication of the interconnect structure, and such removal can be difficult to properly achieve.
Another problem with a polysilicon or oxide hard mask is that typical etches are not selective to these types of hard masks, and where the hard mask is not a conductive material, it tends to oxidize. If the etch etches through the hard mask and underlying layer(s) of a local interconnect, the etch can potentially damage the active area. If so, removal of the hard mask is desirable. One removal method for polysilicon or silica hard mask is a wet etching process. The etching process, however, is not desirably selective to polysilicon or to silica. This results in difficulty in controlling the etch, which difficulty can result in a low yield rate due to an over etching into an underlying active area in the semiconductor substrate.
Typically, refractory material nitrides are used as a barrier layer. For example, the nitrides include titanium nitride and tungsten nitride. It is difficult to stop an etch on a nitride. If the etch is too deep and goes through beyond the nitride, the active area can be compromised. Titanium nitride, as such, is not a preferred material for a dry etching process for a selective etch. To compensate, a type of material is put down that can be patterned, such as a photoresist layer through which the titanium nitride can be selectively removed without a hard mask. It is difficult, however, to control the etch of the titanium nitride without any kind of hard mask, and such difficulty of control may result in an etch through the titanium nitride layer and into the underlying active area.
It may be desirable to use two etchants, such as one to etch a first interconnect material and another to etch the titanium nitride in contact therewith. For example, if titanium nitride and titanium silicide are the layers of the interconnect structure, the titanium silicide can be etched with an hydrofluoric chemistry etch, but the etch will not substantially etch the titanium nitride. Consequently, a second etch is needed that will etch the titanium nitride, where the second etch is selective to titanium silicide.
It would be advantageous to overcome the problems of poor step coverage and uncontrolled etching, and to be able to use one etchant to remove both the interconnect material and an associated barrier layer. In addition, it would be advantageous to utilize a material which also provides an improved step coverage in the fabrication of the interconnect structure.